1. Field of the Invention
The invention relates to arrangements for minimizing multiplexing and demultiplexing (mux/demux) costs, especially in ultra long haul (ULH) all-optical networks. More specifically, the invention is directed to methods of rearranging traffic segment processing order and/or pre-reserving some mux/demux group slots, so as to minimize a total number of mux/demux groups.
2. Related Art
After many years of research and industry efforts, ultra long haul (ULH) technologies for dense-wavelength-domain-multiplexing (DWDM) transport have become more mature, and carriers are deploying such technology for fast provisioning and capital savings. (See A. Chiu and Chang Yu, “Economic Benefits of Transparent OXC Networks as Compared to Long Systems with OADM's,” OFC2003, Atlanta. See also A. F. Wallace, “Ultra long-haul DWDM: network economics,” OFC 2001, TuT1-T1-2 vol. 2. These documents, as well as all other documents cited in this specification, are incorporated herein by reference in their entirety.)
An ULH all-optical network has a number of nodes for adding/dropping traffic and the links with optical amplifiers (OAs, also know as in-line amplifiers, ILAs) connecting the nodes. The nodes are usually multi-degree optical add/drop multiplexers (OADM) 104 (FIG. 1) with wavelength blocking and/or wavelength selection switching capability. The traditional end terminal (ET, also be called half OADM or Degree-1 OADM) has only one termination point (or called terminal in this disclosure) to serve the spur end of the network. The traditional OADM has two terminals and is referred as Degree-2 OADM. A network junction node with 3 or 4 terminals is a Degree-3 or Degree-4 OADM, also called a photonic cross connector (PXC) 102 (FIG. 1).
A network link terminates within a switching node at a “termination point (TP)”, or simply a “terminal”. Each terminal has proper mux/demux structure for adding/dropping chosen wavelengths. It is possible to use an additional degree in a node to support all add/drop traffic with one terminal. The pros and cons of this architecture are addressed by J. Strand and A. Chiu, “Realizing the Advantages of Optical Reconfigurability and Restoration with Integrated Optical Cross-Connects,” Journal of Lightwave Technology, Vol. 21, No. 11, November 2003. A wavelength connection is added and dropped at the terminals through a pair of optical-electrical-optical (OEO) transponders. If the distance between the add/drop points is beyond the reach of the ULH technology, or a unique wavelength cannot be found along the whole path, a pair of back-to-back transponders or a through transponder can be used in an intermediate node to regenerate and/or convert the wavelength, again via proper mux/demux structures 110 (FIG. 1).
The conventional mux/demux architecture has a fixed mux/demux structure where each slot corresponds to a particular wavelength. Then, to set up a connection between two terminals, not only does a wavelength need to be available along the path, but also the same mux/demux slot must be available at both terminals.
The mux/demux structure represents a significant fraction of the total network cost. To reduce both initial capital outlay and subsequent growth cost, the mux/demux structure usually is divided into mux/demux “groups” that can be installed subsequently when needed, as illustrated in blowup 110 (FIG. 1). In general:
  N  =      W    K  
in which:                N is the total number of mux/demux “groups” (typically, N is 2, 4, 8 . . . );        W is the total number of wavelengths per fiber (for example, 80, 160, . . . ); and        K is the number of wavelengths per mux/demux “group” (for example, 10, 20, 40, . . . )        
Deploying the mux/demux structure in groups poses no planning problem for the traditional point-to-point long haul (LH) systems: One simply deploys the same mux/demux group at both end terminals as the existing ones exhausted. For LH systems with one or two OADM between the two end terminals, one typically assigns a wavelength-band for OADM traffic; that is, the OADM can only add/drop wavelengths from a pre-determined wavelength-band. Using LH systems to support nationwide traffic, all wavelengths must be regenerated at the end terminals. Therefore, wavelength planning is confined within each LH system between two end terminals. Wavelength planning is not a problem as long as the number of OADM between two end terminals is small.
However, for ULH networks having dozens of multi-degree OADMs, minimizing wavelength blocking and regeneration becomes a challenging problem. To simplify the problem, conventional wavelength constraint studies (see P. Raghavan and E. Upfal, “Efficient routing in all-optical networks,” Proc. 26th ACM Symp. on Theory of Computing, 1994, New York, pp. 134-143) assume “any-wavelength-to-any-node” deployment requires a full mux/demux structure at every terminal. Undesirably, this assumption represents a substantial increase of initial capital cost. It is estimated that the total mux/demux cost could reach 60% of the total initial equipment capital—more than the combined cost of all optical amplifiers, wavelength blockers, and wavelength selection switches in the network. In a typical nationwide network, a large fraction of terminals are minor terminals (only one mux/demux group is enough) with a few add/drop wavelengths since most of the wavelengths are express or “through” wavelengths. No carrier has the luxury to deploy a full mux/demux structure at every terminal of each node, just for the convenience of wavelength assignment.
The problem of wavelength assignment to reduce mux/demux cost disappears if the mux/demux architecture has “full tunability”—that is, if each slot on the mux/demux structure can be tuned to any wavelength. One simply installs mux/demux groups with enough slots for the number of add/drop wavelengths. However, the per-slot cost in a fully tunable mux/demux structure is much higher than that in a fixed mux/demux structure.
To compromise between performance and cost, some DWDM suppliers offer “partially-tunable” architectures where the W wavelengths are divided into M wavelength bands, and a slot on a mux/demux group is tunable within the corresponding wavelength band. “Partially-tunable” architectures provide some flexibility in wavelength assignment. However, to support an any-wavelength-to-any-node deployment approach, one still needs at least one mux/demux group per tunable wavelength-band in every terminal that increases the total mux/demux equipment cost.
Applicants have realized that, similar to the fixed mux/demux architecture, proper wavelength assignment heuristic algorithms can reduce the number of mux/demux groups in the partially tunable mux/demux architecture. It may become justifiable to deploy the partially tunable mux/demux architecture even if the per-slot cost is higher than that of the fixed mux/demux architecture.
A wide variety of schemes of assigning wavelengths have been proposed.
U.S. Pat. No. 5,963,348 (Oberg) discloses a method for assigning wavelengths in an optical bus network, the network having two pairs of optical fibers.
U.S. Pat. No. 5,999,288 (Ellinas et al.) discloses a method for systematically selecting a wavelength assignment for node pair connections without violating “color clash” (wavelength conflict) rules.
U.S. Pat. No. 6,466,343 (Lahat et al.) discloses a system for assigning optical wavelengths in a wavelength division multiplexing (WDM) switch, the method involving a request table storing requests from plural interface cards.
U.S. Pat. No. 6,529,300 (Milton et al.) discloses an interface at each node in a network performing the basic functions of dropping a band associated with the node, adding a band carrying traffic for another node, and passively forwarding other bands through the node.
U.S. Pat. No. 6,532,090 (Doerr et al.) discloses a wavelength selective cross-connect (WSC) that involves a set of wavelength interchange modules that allows the WSC structure to be simplified.
U.S. Patent Application Publication No. 2003/0194234 (Sridhar et al.) discloses method for dynamic wavelength assignment that adds a minimum number of optical/electronic and electronic/optical (OE/EO) interfaces.
U.S. Patent Application Publication No. 2004/0028410 (Doh et al.) discloses a method for assigning a minimum number of required wavelengths in a WDM ring network.
U.S. Patent Application Publication No. 2004/0208558 (Roorda et al.) discloses a method of wavelength routing in a metro network subtended off a wavelength switched (agile) core network.
From the foregoing survey of conventional arrangements, it is apparent that conventional artisans have not paid adequate attention to the question of reducing costs of mux/demux groups in assigning wavelengths, especially in ultra long haul (ULH) optical networks. Thus, none of the conventional arrangements appear to have solved the problems described above, and there is a need in the art for an arrangement that reduces the costs in networks by assigning wavelengths in such a manner as to minimize the number of mux/demux “groups” required. There is a need for practical wavelength assignment heuristics for deploying fixed mux/demux structures (“groups”) as needed, and without constraining the future network growth. An effective heuristic could significantly reduce capital costs, both initially and over the entire network life cycle.